There has been renewed interest in finding novel ways to interact with devices through the use of multi-touch displays, particularly with mobile devices. Such interaction is driven by gestures. For instance, dragging is a gesture that can move a large view within a view port. This is useful to display documents that are too large for devices with small displays, such as a mobile device. Zooming can be performed by means of pinching gestures. A tap opens a selection or sets the zoom level such that the view port focuses on the tapped part of the document. However, with so many basic gestures being allocated already to essential and frequently needed interactions, multiple selection tasks become cumbersome.
Advances in mobile devices have allowed such devices to render web pages just like an ordinary browser. Users can navigate a large page with ease despite the device's constraint size using a set of simple and intuitive gestures. They can drag and flick a large page to position it within the display view port, pinch the display to zoom in and out of the viewed page or tap on page elements to center and scale the page to fit the view port.
We refer to these gestures as primary gestures, as they are used consistently in many applications, for instance photo browsing, map navigation, document reading and scrolling of data items organized in tables. The gestures and the functions they invoke are essential and defining interaction design principles of the device.
The downside is that these essential gestures are not easily available for other important, but less frequent, tasks. For instance, most WIMP systems implement dragging, dropping and multiple selection with the mouse equivalents of these gestures whereas scrolling and zooming is accomplished through interaction with special widgets (e.g., scroll bars) or additional hardware (e.g., a scroll wheel). Laptops with touch pads are an exception as they already support two finger gestures for scrolling.
While less frequent, filing and editing tasks are also of importance for mobile device applications. For instance, when attaching photos to an e-mail, the familiar photo browser is invoked. In order to select photos, the primary gestures must be redefined based on context, or additional input states must be synthesized. The first choice is a break of consistency and also makes the original functionality of the primary gesture unavailable (e.g., navigating within the photo collection by flicking, and enlarging a photo thumbnail by tapping on it). This has a detrimental effect on the user experience particularly if multiple selection is interspersed with navigation because the meanings of primary gestures change frequently and the change of mode requires additional interaction. The second choice may be difficult to accomplish or have limited usability. For instance, if timeouts are used to synthesize additional input states, then gestures in that state are penalized with a delay. This can be observed in application launchers where tapping and holding an application icon switches into a different mode that allows one to delete the icon or drag it to a different position. Fortunately, this is an infrequent operation and therefore the delay is tolerable. However, for more frequent operations, particularly operations that alternate with navigation tasks, the mode switching delay easily becomes intolerable.
Alternatively, a hardware mode switching button may be used. However, a hardware button is a costly resource. A virtual button, on the other hand, consumes valuable display real estate.
A crossing-based interface along the bounds of the display is a viable solution. Users can invoke actions with swiping gestures across a section of the display side from the outside to the inside. This approach requires minimal display real estate and a number of actions can co-exist conflict-free with primary gestures. We illustrate the concept below.
Crossing-based interfaces, such as the one described by Accot and Zhai in “More Than Dotting The I's—Foundations For Crossing-Based Interfaces”, In Proc. CHI 2002, were suggested as an alternative interaction paradigm to pointing tasks. Crossing tasks also follow Fitt's Law (albeit with different parameters) and in some situations they can be more efficient than pointing tasks. Consider a button versus a line: in the button pointing task the cursor must land on the button and therefore the task time depends on the size of the button. There is no landing zone for the crossing task and therefore it can be faster e.g., if the line is perpendicular to the distance vector between the line and the cursor. Another advantage is that a well positioned line requires significantly less display space than a button for similar performance characteristics.
Pointing and crossing tasks were compared, and as a proof of concept of the crossing interaction principle, Apitz and Guimbretiere in “Crossy: A Crossing-Based Drawing Application”, UIST'04: Proceedings of the 17th Annual ACM Symposium On User Interface Software And Technology, built a sketching tool entirely comprised of crossing widgets. Crossing-based interaction has also been used in the design of interactive menus such as FlowMenu, as described by Guimbretiere and Winograd in “Flowmenu: Combining Command, Text, and Data Entry.” In Proc. UIST 2000, and Control Menu, as described by Pook et al., in “Control Menus: Execution And Control In A Single Interactor.” In Proc. CHI 2000.
In “Laser Pointer Interaction Techniques Using Peripheral Areas Of Screens.”, In AVI'06: Proceedings Of The Working Conference On Advanced Visual Interfaces, Shizuki et al. describe a crossing-based interaction technique that allows a user to control tasks or applications such as a slide presentation or a simple drawing tool with a laser pointer. For instance, a swipe from the right outside of the slide to the inside selects the next slide. The movement of the laser pointer with respect with the slide projection is tracked using a camera.
Froehlich et al. in “Barrier Pointing: Using Physical Edges To Assist Target Acquisition On Mobile Device Touch Screens.” In Assets '07: Proceedings Of The 9th International ACM SIGACCESS Conference on Computers and Accessibility describe a technique they call barrier pointing, which relies on physical elevated edges around the device display to aid to positioning tasks. Selection targets are positioned along the elevated display edges and corners such that the physical edges can be used as stabilizing guides. Selections are confirmed e.g., by lifting a stylus while it is within the target's boundaries.